1. Field of the Invention
This invention relates to a method and apparatus for effecting the perforating and the gravel packing of a production zone in a subterranean well.
2. Description of the Prior Art
As oil and gas wells are drilled to constantly increasing depths, the cost of completion or workover of a well is disproportionally increased by the number of trips of completion apparatus that must be made into the well in order to effect its completion or workover. Necessarily, every encased producing well has to have the casing perforated in the production zone. It is equally necessary in the case of many wells to provide gravel packing in the area of the perforations to filter out sand produced with the production fluids and thus prevent its entry into the well bore and into the production conduit. It has heretofore been necessary to make several trips of a work string into the well in order to first effect the perforation of the well casing and then the gravel packing of one or more production zones surrounding the perforations. Most commonly used tubing conveyed perforating apparatus rely upon percussion firing of explosive charges. Such firing is produced by dropping a weight through the tubular work string to fire a primer carried by the perforating apparatus located at the bottom of the well. It is therefore necessary that the bore of the tubular work string be initially unrestricted, at least to the extent to permit the free passage of the firing weight or bar therethrough.
It has previously been suggested that the gravel packing of a plurality of production zones of a well could be accomplished in a single trip of a specially designed gravel packing apparatus into the well. Such apparatus is, for example, disclosed in U.S. Pat. No. 3,987,854 to Callihan et al. and also in the copending application Ser. No. 170,494, filed July 21, 1980, and assigned to the assignee of the present application. In both instances, however, the crossover tool which forms an essential part of such multiple zone gravel packing apparatus, has not provided an unrestricted axial passage through the crossover apparatus. Therefore, it has been a practical impossibility to enter the well with both a perforating apparatus and a gravel packing apparatus and accomplish both operations in the same trip.
Additionally, since the entire perforating operation is to be performed in the same single trip, it is highly desirable that the perforating gun be capable of adjustment in the field of both the total number and the horizontal and vertical spacings of the shaped charges employed in the perforating operation. An economical apparatus permitting the convenient field assembly of a plurality of shaped charges in any desired vertical and horizontal configuration has not heretofore been available.